Making an aluminum nitride heater

ABSTRACT

A method of making a heater includes an aluminum nitride base having equal to or less than 1% impurities, particularly one embodiment having none of polybrominated biphenyl, polybrominated diphenyl ether, hexabromocyclododecane, polyvinyl chloride, chlorinated paraffin, phthalate, cadmium, hexavalent chromium, lead, and mercury. The base is fired in a heating unit before any layering. Thereafter, on a topside and backside of the base a conductor layer is layered and allowed to settle and dry before firing. Next, a resistive layer is layered on the base from a resistor paste such that the resistive layer connects to the conductor layer on the topside. The resistor paste is allowed to settle and dry and then the base with the conductor and resistor layers is fired. At least four layers of glass are layered next over the resistive layer, each instance thereof including layering a glass, drying the glass and firing.

This application claims priority as a Divisional Application of U.S.Application Serial No. 16/782,287, filed Feb. 5, 2020.

FIELD OF THE INVENTION

The present disclosure relates to a heater for a variety of uses. Theheater defines an essentially pure aluminum nitride base having thickfilm printing, including resistive and conductive layers and overlayersof glass.

BACKGROUND

Heaters have many and diverse applications in the industries ofautomotive vehicles, appliances, and consumer items, to name a few. Theyserve as cabin heaters in electric and hybrid vehicles, water heatersfor dishwashers and washing machines, and sources of heat for hairdryers/straighteners, cooking apparatuses, and space heaters, etc. Inimaging devices, such as printers and copiers, heaters exist in fusersto melt and fix toner on media. However, when narrow media (e.g.,envelopes) are imaged, portions of the fusers extending beyond the mediaquickly overheat due to low thermal mass components which can becomedamaged. Some printers having a belt fuser heated by an aluminum oxide,thick-film printed, ceramic heater slow from 70 pages-per-minute printspeed for full-sized media to 7 ppm for narrow media to prevent fuserdamage by better controlling temperature, but at the expense of printingspeed.

Positive Temperature Coefficient (PTC) heaters have been tested as ameans of preventing damage to fusers during printing of narrow media.Some have suggested that PTC heaters are desirable for 2 mainreasons: 1) PTC materials have a Curie point in a temperature regionthat can eliminate damage to other components, theoretically allowingnarrow media to be printed at higher speeds than occurs with aluminumoxide heaters; and 2) the lower Curie point of the PTC materials servesas a safety feature. However, the inventors note that PTC materialsalone are not advantageous because they have relatively low thermalconductivity. For PTC materials to be more effective, heat must bedissipated quickly from the PTC materials. Furthermore, PTC materials,when cold, have a severely low resistance - creating an extremely highin-rush current.

An aluminum nitride substrate, thick film heater, has been alsomentioned for preventing damage to fusers during printing of narrowmedia. However, aluminum nitride heaters to date have been limited tohybrids consisting of about 80% aluminum nitride and about 20% aluminumoxide. Such materials have a thermal conductivity of about 80 W/mKcompared to aluminum oxide which has a thermal conductivity of about 23W/mK. Thus, the inventors propose an essentially pure aluminum nitridesubstrate, thick film heater, having a significantly higher thermalconductivity with a thermal conductivity of about 200 W/mK. No one as ofyet, however, has been able to successfully thick-film print a largearea resistor heater on such a substrate without Kirkendall voids (e.g.,voids or gaps between the thick-film printed conductor and resistorlayers on the substrate). In turn, voids such as these lead to extremelyhigh electrical resistance causing heater failure. Furthermore, existingglass pastes cannot be used for thick-film printing di-electric glassesor cover glasses serving as electrical insulation layers over theconductor and resistor layers due to large voids in the glass. Theinventors identify one reason for this as current manufacturingprocesses have difficulty in out-gassing nitrogen gas during hightemperature firing/heating (typically 850° C.) of the substrate havingthe conductor, resistor, and glass paste materials.

A need exists to overcome the foregoing. The inventors further note thatany solutions in the technology of heaters should further contemplatethe competing design constraints found in power consumption, safetyfeatures, warm-up characteristics, operating temperatures, heatingspeeds, thermal conductivity, materials, costs, electrical requirements,construction, materials to-be-heated, temperature control,installation/integration with other components, size, shape, anddimensions, and the like.

SUMMARY

A heater includes an aluminum nitride base having equal to or less than1% impurities, especially none of polybrominated biphenyl (PBB),polybrominated diphenyl ether (PBDE), hexabromocyclododecane (HBCDD),polyvinyl chloride (PVC), chlorinated paraffin, phthalate, cadmium,hexavalent chromium, lead, and mercury. At least one longitudinallyextending resistive trace of silver and palladium overlies the base asdoes a conductor of silver and platinum or silver and palladium thatelectrically connects to the resistive trace to apply an externalvoltage to the resistive trace for heating thereof. At least four, butoptionally five layers of glass overlie the resistive trace andconductor, but not an entirety of the conductor. A first two consecutivelayers of the glass layers define a first glass having a solid contentof more than 65% and a viscosity of 100 Pa · s or less. The followingtwo or three consecutive layers of the five layers define a second glassdissimilar to the first.

A method of making a heater includes thick film printing an aluminumnitride base having equal to or less than 1% impurities. The base isfired in a heating unit before any layering of the base. Thereafter, ona topside and backside of the base a conductor layer from a conductivepaste is layered and allowed to settle and dry before firing the basehaving the conductor layers. Next, a resistive layer is layered on thebase from a resistor paste such that the resistive layer connects to theconductor layer on the topside of the base. The resistor paste isallowed to settle and dry and then the base with the conductor andresistor layers is fired in the heating unit. At least four layers ofglass are layered next over the resistive layer. Each instance thereofincludes layering a glass, drying the glass and firing the baseincluding the glass so layered. Settling, drying, and firing profilesare also noted under various embodiments.

The embodiments noted herein eliminate life-limiting Kirkendall voids inthick film printing of silver platinum (AgPt) conductors and silverpalladium (AgPd) resistors at the interface of the conductor andresistor layers. The embodiments also eliminate large, prohibitive voidsin layers of glass.

Further embodiments teach blends of resistor paste of about 80% silverand about 20% palladium for thick film printing on an aluminum nitridebase, but still supporting a 115 volt resistor. The embodiments pioneerthe use of relatively low palladium content unlike typical prior pastesof about 45% silver and 55% palladium. Pastes for conductor layersinclude content of about 93% silver and about 7% palladium or platinumand being essentially free of Kirkendall voids at the juncture of theresistor to conductor. The embodiments overcome problems noted withtypical prior conductive pastes.

Still further embodiments note the use of a relative low firingtemperature for a thick film di-electric (“cross glass”) and a coverglass overlying the resistor and conductive layers. As has beenpracticed in the past, typical large area ceramic heaters utilized aglass firing temperature of approximately 850° C. Whereas, embodimentsof the present disclosure separate the “cross glass” (higher di-electricproperties) from a cover glass (lower di-electric properties butimproved surface quality) and fire the cross glass at a peak temperatureas low as about 835° C. while the cover glass could be fired as low asabout 810° C. Without being bound by theory, the lower firingtemperatures are believed to be fundamental to preventing large voids inany of the glass layers (leading to poor di-electric strength) andguaranteeing no Kirkendall voids between the resistive and conductivelayers. The inventors also found the lower temperatures of firing theglass helped in two ways. First, silver migration was diminished betweenthe silver content of each of the resistor and conductor material.Second, the oxidation of the palladium in the resistor and conductormaterials was lowered in those embodiments containing palladium, wherepalladium oxidation and reduction rates are known to relate toKirkendall voids.

In even further embodiments, oxidizing or plasma treating the surface ofthe aluminum nitride base further contributes to eliminating thedeleterious effects of nitrogen out-gassing during later instances offiring the base which occurs during print, dry, and firing sequences ofthick film printing.

The embodiments of the invention support many forms of heaters for usein many and diverse applications. In one design, the heater iscontemplated for use as a cabin heater for Electric Vehicles (EV) andhybrid vehicles. Traditional cabin heaters today utilize a series of PTCheaters embedded in a radiator type arrangement, i.e. an array ofaluminum finned heat spreaders attached to PTC heaters. Air flowsthrough the aluminum fin heat spreaders - removing heat from the PTC.However, a major disadvantage of a PTC heater (only) arrangement is thatit heats relatively slowly. Auto users complain about the relativelyslow warm-up time of the PTC only cabin heaters which is attributed to arelatively high in-rush of current. Developers then must be careful notto drain the battery of the vehicle or use excessive fuel to handle theexcessive in-rush current. With the embodiments of the present heater,however, the essentially pure aluminum nitride heater supports thermalconductivity of about 200 W/mK and can serve as both a pre-heater andheat spreader to the PTC heater array - eliminating any slow warm-up ofcabin heaters. It also supports power designs of about 1200 Watts ormore.

In other applications, an essentially pure aluminum nitride heaterfacilitates the printing of narrow media in an imaging device withoutthe problems of the prior art and can bear the duties of warming up abelt and back-up roller in a belt fuser in imaging devices. Such aheater can also pre-heat a series of PTC elements and then be turnedoff. Heating of the PTC can commence until the in-rush current is within10% - 20%, for example, of the steady state current. Such a dualcombination heating scheme allows for much quicker warm-up times.

BRIEF DESCRIPTION OF THE FIGURES:

FIG. 1A is an exploded view of an aluminum nitride heater according to arepresentative embodiment of the present invention;

FIG. 1B is a non-exploded view of the aluminum nitride heater of FIG.1A;

FIG. 1C is a backside view of the aluminum nitride heater of FIG. 1B;

FIGS. 2A-2F are diagrammatic views of a representative sequence ofprinting, drying and firing layers on a substrate when forming analuminum nitride heater;

FIGS. 3A-3E are diagrammatic views of a representative sequence ofprinting and drying layers on a substrate when forming an aluminumnitride heater;

FIGS. 4A and 4B are diagrammatic views of a representative sequence ofonly firing a substrate with an optional layer(s) when forming analuminum nitride heater;

FIGS. 5A-5E are diagrammatic views of a representative sequence ofprinting, drying and firing patterned layers on a substrate, such asresistive traces and conductors, when forming an aluminum nitrideheater;

FIGS. 6A-6I are diagrammatic planar views of a representative sequenceof patterning a plurality of top layers on a base forming an aluminumnitride heater;

FIG. 7 is a diagrammatic view of a large substrate for dicing intoplural, individual aluminum nitride heaters; and

FIG. 8 is a graph of a representative heating profile according toembodiments of the invention for firing in a heating unit a base orsubstrate with or without overlying layers.

DETAILED DESCRIPTION

FIGS. 1A and 1B teach a heater 10 for a variety of uses. The heaterincludes an essentially pure aluminum nitride base or substrate 12.Essentially pure is at least 5% impurities or less, but equal to or lessthan 1% is preferred. In one embodiment, the impurities of the base donot include any of polybrominated biphenyl (PBB), polybrominateddiphenyl ether (PBDE), hexabromocyclododecane (HBCDD), polyvinylchloride (PVC), chlorinated paraffin, phthalate, cadmium, hexavalentchromium, lead, and mercury. The shape of the base includes alongitudinally extending solid of a generally rectangular shape having alength (1) and width (w) dimension and a thickness (t). After separatingby dicing in FIG. 7 from a saw 15 along dashed lines 17 and 19 from alarger wafer 20, for example, representative dimensions of each heater10 include a thickness in a range of about 0.5 - 0.7 mm, a length in arange of about 150 - 160 mm, and a width in a range of about 6-8 mm.

With continued reference to FIGS. 1A and 1B, each heater 10 includes atleast one resistive trace 22 on a topside 24 of the base. Connected toeach resistive trace at interface 25 is a conductor 26. During use, theconductor 26 receives power from an external voltage source to power theresistive trace(s) 22. In turn, the resistive trace heats and providesheating to the device in which it is used, such as for a cabin heater inan electric or hybrid vehicle or a fuser in an imaging device. In oneembodiment, the external source is 115 VAC. In others, it is 12 VDC, 350VDC, 650 VDC or 800 VDC. In any, the resistive trace and conductorsupport the voltage and lack Kirkendall voids at the interface 25, byway of the methods of manufacturing the heater as described below. Indimensions, the thickness of the resistive trace is about 10 - 13 µm onthe aluminum nitride base and has a length of about 135 - 145 mm and awidth of about 4.5 - 5.5 mm. The conductor has a thickness of about 9 -15 µm on the aluminum nitride base, a length of about 11 - 13 mm, and awidth of about 4.8 - 5.8 mm. Also, the resistive trace has a resistanceof about 10 - 12 ohms at 195° C. The resistive trace is formed from aresistor paste of about 80% silver and 20% palladium while the conductoris formed from a conductive paste of silver and palladium or platinum.In one embodiment, pastes for conductor layers include content of about93% silver and about 7% palladium or platinum.

Overlying each resistive trace and at least a portion of the conductor,but not an entirety of the conductor as it needs to connect to theexternal power source, is at least four layers of glass 30 (30-1, 30-2,30-3, 30-4, FIG. 1 a ). The glass is any of a variety but the first twoconsecutive glass layers 30-1, 30-2 are of a first type, while the nexttwo 30-3, 30-4 are of a different type. The first type defines a crossglass layer, while the different type defines a cover glass layer. Anyof the four glass layers define a glass having a viscosity of 100 Pa · sor less. More particularly, the viscosity exists at 90 Pa · s or less,especially 65 Pa · s or less. Its solid content, on the other hand,exists at 65% or more. In various specific embodiments, the glass ispurchased commercially from AGC, Inc. (formerly the Asahi Glass Company)as seen in Table 1. Its properties are also noted.

TABLE 1 AGC, Inc. Glass Paste ID Thixotropic Index Viscosity (Pa · s)Solid Content (%) AP5717B10 2.0 - 2.4 100 66 AP5717B13 1.6 89 69AP5717B14 1.4 61 72

In any layer of glass, the dimensions include a thickness in a range ofabout 10 - 13 µm on the aluminum nitride base, a length in a range ofabout 135 - 145 mm, and a width in a range of about 4.5 - 5.5 mm. In oneembodiment, the first two consecutive layers 30-1, 30-2 of the at leastfour glass layers together have a thickness of about 24 µm. The next twoconsecutive layers 30-3, 30-4 and a fifth layer of glass (not shownuntil FIG. 6I) together have a combined thickness of about 65 µm. Thefifth layer of glass also overlies the base and resistive and conductivelayers and is similar in composition to any of the cover glass layers.

With reference to FIG. 1C, a bottom or backside 40 of the base 12optionally includes one or more thermistors 50. They interconnect with asame or different conductor 26 of the topside. They are positioned tomeasure the temperature of the heater 10 and the conductor 26 connectsthe thermistors to external sources to measure, store and control thetemperature.

With reference to the Figure sets of 2A et seq., 3A et seq., and 4A etseq., the general process steps for fabricating the heater 10 of FIGS. 1a and 1 b will be described. They include one or more of thick-filmprinting, settling, drying, and firing or heating. As shorthand from theindustry, they are generally known as print, dry, and fire, or PDF.

In more detail, the FIGS. 2A-2F show printing, drying, and firing. InFIG. 2A, a base or substrate, such as the essentially pure aluminumnitride base 12, is provided. In FIG. 2B, thick-film printing of thesubstrate includes providing a mesh stencil 60 upon and through which apaste 62 is applied. In the instance of layering a resistor, conductoror glass, a resistive paste, a conductive paste or a glass paste isapplied. In FIG. 2C, a leveling device 64, such as a squeegee or otherscraper, levels the paste on a surface 66 (FIG. 2B) of the base. In FIG.2D, the paste so applied is allowed to settle on the base forming alayer 70 upon removal of the stencil. This settling occurs typically forabout five to ten minutes at room temperature, e.g., 20° - 25° C. InFIG. 2E, the base and layer is provided to a curing or drying unit 80.The drying unit typifies a box oven or blast furnace and the base isprovided to the unit along a conveyor, typically. The drying unit beginsdrying the layer 70 at around room temperature followed by a curing ordrying cycle of about 30 minutes reaching temperatures of 140°- 160° C.In one embodiment, the drying cycle includes applying infrared heat orhot air (both given generically as heat 82) for a period of time ofabout 30 total minutes at a temperature profile of the drying unitbeginning at about 25° C. and ramping up to about 80° C. for about 10minutes, ramping up again to about 160° C. for about 10 minutes andcooling down to below 50° C. After that, the base 12 with layer 70 isfired in a heater or firing unit 80′. In some instances, the firing unit80′ is the same unit as the drying unit 80, but having different heatingprofiles. In others, the firing unit 80′ is different from the dryingunit 80 and the base advances from one unit to the next along aconveyor, typically. In any, the heating profile for heating the basedepends upon which type of layer is most recently printed and driedthereon, e.g., resistive layer, conductive layer or glass layer.

In FIG. 8 , a representative heating profile for any layer is shown ingraph 100. Namely. the heating profile for a un-layered base orresistive or conductive layer is shown by the solid line 102, whereas adashed line 104 depicts the heating profile for glass. In general, theheating profile of the heating unit includes a total heating time ofabout 40 total minutes starting at about 25° C. and ramping up to a peaktemperature (part of zones 5-8) by 20 minutes and maintaining the peaktemperature for at least 10 minutes and decreasing the temperature ofthe heating unit (post zone 8) for at least 10 minutes thereafter.Cooling continues even further thereafter (post zone 12) untilcompletely cooled. For an un-layered base or the resistive or conductivelayers, the peak temperature reaches about 850° C. The glass layers, onthe other hand, have a peak temperature of 830° C. or 810° C., dependingon which layers. Table 2, infra, provides a representative embodiment ofwhich glass layers heat at which temperatures.

With reference to FIGS. 3A-3E, instances are shown of thick-filmprinting a base 12 to form and dry a layer 12 thereon. The views aresimilar to FIGS. 2A-2E, except there is no instance of firing thebase/layer(s) in a firing unit. Rather, the processing steps onlyinclude printing and drying. Similarly, too, FIGS. 4A and 4B show themere firing of a substrate 12 in a firing unit 80′, but without anyinstance of printing or drying a layer on the base or substrate.

With reference to FIGS. 5A-5E, a sequence of events depicts theprinting, drying and firing steps of processing, but for a patternedlayer overlying a base. That is, FIG. 5A shows a base 12. In FIG. 5B,the mesh stencil 60′ includes a patterned layout 61 for receiving (1) apaste 62 and leveling (2) therein by the leveling device 64, but whereasa remainder of the stencil includes a masked portion 65 preventingapplication of the paste 82 to the base 12. In FIG. 5C, the result isgiven with a base 12 having patterned layers thereon. In this instance,two longitudinally extending resistive traces 91 reside on the surface66 of the base 12 in the pattern matching the patterned layering 61 ofthe stencil 60′. Of course, any patterned shapes are possible. Settlingof the patterned layer then occurs for about five to ten minutes at roomtemperature and are similar to that of FIG. 2D. Heating of the base andpatterned layers next occurs in FIG. 5D, including either curing and orfiring in a drying and or heating unit 80/80′. In FIG. 5E, the patternedlayer of the base 12 is further shown with another patterned layer 93representing a conductive layer connected to a resistive layer at aninterface 25. Again, any patterning of layers is contemplated herein.

With the principles of any instances of printing, drying and firing on abase, reference to FIGS. 6A-6I show one embodiment of forming analuminum nitride heater according to the invention. At FIG. 6A, anessentially pure aluminum nitride base 12 is provided. The base has 5%or fewer impurities, especially 1% or less. A surface 66 of the base isoptionally pretreated by oxidizing the surface or providing a plasmatreatment according to known techniques. The base is then firedaccording to the heating profile 102 of FIG. 8 , up to a peaktemperature of 850° C.

In FIG. 6B, a conductor layer 26 is patterned on a topside 24 of thebase by thick-film printing and drying. The conductor layer is formedfrom a conductive paste. The past is a blend of silver and platinum orsilver and palladium. The silver comprises more than 90% of the paste.In one design, the paste is about 93% silver and about 7% palladium. InFIG. 6C, on a backside 40 of the base 20, another conductor layer 26 ispatterned by thick-film printing and drying. The paste is the same asthe topside paste and the backside is used to secure thermistors, e.g.,FIG. 1C, such as by resistance-welding thermistors to the conductorlayer. Thereafter, the base 12 with top and backside conductor layersare fired. The firing takes the form of the heating profile 102 of FIG.8 and reaches a peak temperature of about 850° C. In alternateembodiments, the processes of FIGS. 6B and 6C could be reversed with thelatter occurring first.

In FIG. 6D, a resistive trace 22 is patterned and connects to theconductor layer 26 at an interface 25. The trace is formed by thick-filmprinting with a patterned stencil and allowed to settle into place atthe interface whereupon it is dried. The trace, formed also of a blendof silver and palladium, representatively comprises 80% silver and 20%palladium. Thereafter, the trace together with the base and the top andbackside conductor layers is fired in a firing unit. The firing takesthe form of the heating profile 102 of FIG. 8 and reaches a peaktemperature of about 850° C.

In FIG. 6E, a first glass layer 30-1 is patterned over the resistivetrace and portions of the conductor. The first glass layer is patternedby thick-film printing, then dried and fired. The heating profile takesthe form of the dashed line 104 in FIG. 8 and reaches a peak temperatureof about 830° C. The glass layer is typified as a cross glass layerformed from a paste sold by AGC, Inc. as AP5717B14. It has thixotropicindex of 1.4, a viscosity of about 61 Pa · s and a solid content of morethan 70%, especially 72%. Similarly, in FIG. 6F, a second glass layer30-2 is patterned over the first glass layer 30-1 and also covers theresistive trace and portions of the conductor. The second glass layer ispatterned by thick-film printing, then dried and fired. The heatingprofile takes the form of the dashed line 104 in FIG. 8 and reaches apeak temperature of about 830° C. The second glass layer is also a crossglass layer formed from a paste sold by AGC, Inc. as AP5717B14.

In FIG. 6G, a third first glass layer 30-3 is patterned over the secondglass layer and resistive trace and portions of the conductor. The thirdglass layer is patterned by thick-film printing, then dried and fired.The heating profile takes the form of the dashed line 104 in FIG. 8 andreaches a peak temperature of about 830° C. The glass layer in thisembodiment, however, is cover glass layer formed from a paste sold byAGC, Inc. as AP5717B13. It has thixotropic index of 1.6, a viscosity of90 or less, about 89 Pa · s and a solid content of about 69. Similarly,in FIG. 6H, a fourth glass layer 30-4 is patterned over the third glasslayer and resistive trace and portions of the conductor. The fourthglass layer is patterned by thick-film printing, then dried and fired.The heating profile takes the form of the dashed line 104 in FIG. 8 andreaches a peak temperature of about 830° C. The glass layer is coverglass layer formed from a paste sold by AGC, Inc. as AP5717B13. It toohas thixotropic index of 1.6, a viscosity of 90 or less, about 89 Pa · sand a solid content of about 69.

In FIG. 6I, a fifth glass layer 30-5 (optional in some embodiments,hence the dashed lines) is patterned over the fourth glass layer andresistive trace and portions of the conductor. The fifth glass layer ispatterned by thick-film printing, then dried and fired. The heatingprofile takes a form similar to the dashed line 104 in FIG. 8 , butreaches a peak temperature lower than any other temperature, at around810° C. The glass layer is cover glass layer formed from a paste sold byAGC, Inc. as AP5717B13. It has thixotropic index of 1.6, a viscosity of90 or less, about 89 Pa · s and a solid content of about 69.

In table form, as a series of processes # 1-11, Table 2 shows the makingof an essentially pure aluminum nitride heater as a technicalspecification. Namely:

TABLE 2 # Process Step Sequence Process Temp (°C) Spec 1 Fire Base F 8502 Conductor Topside PD 3 Conductor Backside PD 4 Fire F 850 5 Resistivetrace PDF 850 6 Uniformity Checked 7 COG1 PDF 830 Total thickness 8 COG2PDF 830 24 microns 9 OG1 PDF 830 Total thickness 65 microns 10 OG2 PDF830 11 OG3 PDF 810 Notes: #1 is optional, #2 and #3 can be reversed, PD= Print, Dry, F = Fire, PDF = Print, Dry, Fire, COG1 = 1^(st) “CrossGlass” layer, COG2 = 2^(nd) “Cross Glass” layer, OG1 = 1^(st) CoverGlass layer, OG2 = 2^(nd) Cover Glass layer, OG3 = 3rd Cover Glasslayer.

Thereafter, upon cooling, the resistive trace of the heater becomestested under voltage conditions of 1.75 KVAC applied to the conductorlayer. Resistance of the trace is tested cold at room temperature andupon heating the heater to about 200° C. Its resistance should be about10 ohms at room temperature and about 11 ohms upon heating. A range of+/- 2 ohms is acceptable.

The foregoing description of several structures and methods of makingthe same has been presented for purposes of illustration. It is notintended to be exhaustive or to limit the claims. Modifications andvariations to the description are possible in accordance with theforegoing. It is intended that the scope of the invention be defined bythe claims appended hereto.

1. A method of making a heater, comprising: on an aluminum nitride basehaving equal to or less than 1% impurities, firing the base beforelayering any layers on the base; layering on a topside and backside ofthe base a conductor layer from a conductive paste and letting dry theconductor layer on each side of the base; firing the base having theconductor layers on each side of the base; layering a resistive layerfrom a resistor paste on the base such that the resistive layer connectsto the conductor layer on the topside of the base and letting dry theresistive layer; firing the base having the resistive layer and theconductor layers on each side of the base; and layering at least fourlayers of glass over the resistive layer, each instance of said layeringthe at least four layers of glass including layering a glass, drying theglass and firing the base including the glass said layered.
 2. Themethod of claim 1, wherein the layering the glass includes layering twoconsecutive layers of a glass having a solid content of 65% or more anda viscosity of 100 Pa ^(.) s or less.
 3. The method of claim 1, whereineach instance of firing the base for the conductor and resistive layersincludes heating a heating unit having the base therein to a first peaktemperature of about 850° C. and for said each instance of said firingthe base including the glass heating the heating unit having the basetherein to a second peak temperature of about 830° C., each instance ofthe heating at one of the first and second peak temperatures occurringfor at least 5 minutes.
 4. The method of claim 1, further includingletting settle the conductor and resistive pastes before said lettingdry the conductor and resistive layers, the letting settle including aperiod of time of about 5 to about 10 minutes at about 20° C. to about25° C.
 5. The method of claim 1, wherein each instance of said lettingdry the conductor and resistive layers further includes drying in adrying unit for a period of time of about 30 total minutes at atemperature profile of the drying unit having the base therein beginningat about 25° C. and ramping up to about 80° C. for about 10 minutes,ramping up again to about 160° C. for about 10 minutes and cooling downto below 50° C.
 6. The method of claim 1, further including providing amesh stencil for each instance of said layering.
 7. The method of claim6, further including leveling through the mesh stencil the conductor andresistor pastes and the glass.
 8. The method of claim 1, furtherincluding oxidizing a surface of the aluminum nitride base or plasmatreating the surface before said firing the base before said layeringany layers on the base.